Added Source Map description

Also cleaned up a bit of organization.
Revert this (new lines after headers). See API Blueprint spec, MSON spec, MSON AST spec. 

Furthermore if , **next time you do want to some global aesthetic change like this, make it a separate PR, do not smuggle it with something different like this.**

 It will also make reviewing the PR much easier.

And no we do not have to unify API Blueprint AST explicitly, just do not open the scissors of inconsistency even more.